Depends on your definition of "locals". There are plenty of Chinese and Chinese-looking people who live in or visit HK and don't speak Cantonese.

Mandarin is very common in HK nowadays, and most people should be able to understand it to soem extent even if they don't speak it too well. Anyway, visitors from mainland China tend to spend a lot of money so it is to retailers' advantage if they can communicate.

If you are speaking to someone who works in a customer-facing role in areas frequented by tourists, English should be fine. If you are worried about a negative reaction to speaking Mandarin, first of all this will not be overt, and probably your accent will give you away as being an overseas Chinese which will have different connotations. You can always start off in English and then explain (in Mandarin) that your Chinese is not so good; or you could start off by asking in Mandarin whether the person you are talking to speaks English,